Add a on_stop job to cleanup as-if-foss branch

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/141492



Merged-by: David Dieulivol <ddieulivol@gitlab.com>
Approved-by: David Dieulivol <ddieulivol@gitlab.com>
Reviewed-by: Lin Jen-Shin <jen-shin@gitlab.com>
Co-authored-by: Lin Jen-Shin <jen-shin@gitlab.com>